This invention is concerned with improvements in or relating to method and apparatus for measuring the frequency response of an element or elements in a digital transmission path. The term "element" where used herein means, for example, an equalizer, a filter, a cable simulator or any combination thereof including, as appropriate, amplifier circuits associated therewith, as is known to those skilled in the art.
In digital transmission systems, receivers and generators frequently include components whose characteristics are varied by a control loop or automatic gain control (AGC) in order to provide automatically compensation for different lengths of cable along a transmission path of the system. This arrangement makes it impossible to measure frequency response of, for example, the equalizer of a regenerator in the normal manner which would be by disconnecting the normal traffic signal and using a network analyzer.
In some cases where the design permits, it may be possible to freeze the control voltage while a signal is applied. The signal can then be removed and a network analyzer connected, but this method still requires access to internal circuit nodes or connectors.
In the particular case of the British Post Office's 2mb/s digital line system, the regenerators incorporate automatic equalization which compensates for different cable lengths between regenerators. The design is such that the overall frequency response of the cable plus the equalizer is flat up to almost half the bit rate. Above this level, the response is rolled off to optimize the signal-to-noise ratio (S/N) at the decision point of the regenerator. What is of interest is to predict that bit error rate in terms of the S/N ratio at the regenerator input, which requires knowledge of the equalizer's relative frequency response. Because the spectra of the noise differ from that of the signal, their powers are affected differently by the equalizer. This changes the S/N ratio.
Equalizers are usually contained within a "black box" so that access is available only to its input but not to its output which is directly electrically connected to flip-flops, schmitt triggers or the like whose output is unsuitable for monitoring. As mentioned above, the general solution has been to break the circuit at the equalizer output or to use a higher impedance robe at this point. Whichever method is adopted, access must be gained to the equalizer and hooking up of the equalizer circuit modifies the circuit itself so that a true picture of its frequency response cannot be obtained. This, if a swept sine wave of constant amplitude is supplied to the input of the equalizer and the amplitude of the signal at its output is measured (for example, by a synchronous detector), the output amplitude does not provide a correct representation because the equalizer does not have a normal signal passing through it and is not, therefore, being tested under normal operating conditions. If the equalizer includes automatic level control or line build out then an appropriate feedback voltage must be supplied. Most regenerators do not include provision for supplying this feedback voltge.